Electrolytic method for producing a colored anodized layer on aluminum and alloys of aluminum

ABSTRACT

The invention is addressed to the coloring of aluminum and alloys of aluminum by anodization wherein the aluminum article is immersed as the anode in an electrolytic bath formulated of an aqueous solution of a naphthalene sulphonic acid or alkali metal salt thereof and sulphuric acid, with or without an aliphatic acid of carbon atoms.

United States Patent Chretien et al.

[ 51 Apr. 25, 1972 ELECTROLYTIC METHOD FOR PRODUCING A COLORED ANODIZED LAYER ON ALUMINUM AND ALLOYS OF ALUMINUM Inventors: Roland Chretien, Paris; Henri Richaud, Le

Mans, both of France Assignee: Pechiney, Compagnle de Produits Chimiques et Electrometallurgiques, Paris, France Filed: Apr. 15, 1969 Appl. No.: 816,420

Related U.S. Application Data Continuation-impart of Ser. No. 495,708, Oct. 13, 1965, abandoned, Continuation-impart of Ser. No. 651,971, July 10, 1967, abandoned.

Foreign Application Priority Data July 13, 1966 France ..69304 Primary Examiner-John H. Mack Assistant Examiner-R. L. Andrews Attorney-McDougall, l-lersh, Scott & Ladd [57] ABSTRACT The invention is addressed to the coloring of aluminum and alloys'of aluminum by anodization wherein the aluminum article is immersed as the anode in an electrolytic bath formulated of an aqueous solution of a naphthalene sulphonic acid or alkali metal salt thereof and sulphuric acid, with or without an aliphatic acid of carbon atoms.

25 Claims, No Drawings ELECTROLYTIC METHOD FOR PRODUCING A COLORED ANODIZED LAYER ON ALUMINUM AND ALLOYS OF ALUMINUM This application is a continuation-in-part of our copending applications Ser. No. 495,708, filed Oct. 13, 1965, and entitled Method for Producing Colored Oxide Layers .on Alloys of Aluminum by Anodic Oxidation and Composition, and Ser. No. 651,971, filed July 10, 1967, and entitled Method for Producing Colored Anodized Layer on Aluminum and Alloys of Aluminum both now abandoned.

This invention relates to the production of colored aluminum oxide layers by electrolytic treatment of aluminum or alloys of aluminum and it relates to the formulation of the electrolytic bath and toproducts produced thereby.

To the present, colored layers have been produced on aluminum and alloys of aluminum by a two stage process in which a colorless oxide of aluminum is first formed on the surface by anodic oxidation in an electrolyte containing sulphuric acid. Thereafter the formed anodized layer of aluminum oxide is colored by the use of suitable colored pigments. From the architectural standpoint, the process described is insufficient by reason of the instability and non-uniformity of the formed colored layer.

Colored layers have also been formed on aluminum and alloys of aluminum in a one stage process wherein use is made of an electrolytic bath formulated to contain an organic acid or salt thereof, when certain conditions are maintained during the electrolysis. For example, additions of oxalic acid to the electrolyte and the use of high current densities will operate to produce an oxide layer having a gold tint when use is made of direct current and a brownish layer when use is made of alternating current. The color intensity is sufficient only if very thick layers on the order of 50 to 600 microns are produced and the layers are generally non-uniform in color.

Electrolytes formulated to contain other organic compounds have also been proposed, such as sulphosalicylic acid in German Pat. No. 657,902, but the colors obtained are weak unless layers of substantial thickness are produced.

It is an object of this invention to produce and to provide a method for producing relatively thin layers of high and uniform color intensity on aluminum and alloys of aluminum by electrolytic treatment and it is a related object to produce a new and improved electrolyte for use in the practice of same.

It has been found, in accordance with the practice of this invention, that a coloration of high color intensity and good uniformity can be obtained on the surface of aluminum and alloys of aluminum by the use of an electrolyte formulated to contain a mixture of sulphuric acid and 2-naphthol-6, 8- disulphonic acid (G acid), 2-naphthol-3, 6disulphonic acid (H acid), 1, 8-dihydroxynaphthalene-3, 6-disulphonic acid (chromatropic acid) or 1-naphth0l-8-amino-3, 6-disulphonic acid. instead of making use of one or more of the described free sulphonic acids, use can be made of the ammonium, alkali metal or other water soluble salts thereof. Anodic treatment in the electrolyte will give a fine gold color to aluminum or a color ranging from bronze, beige, grey or chestnut to alloys of aluminum, depending somewhat upon the composition of the alloy.

in practice, the composition and conditions can be varied over fairly wide limits. The naphthalene sulphonic acid component can be employed in solution in the aqueous electrolytic bath in an amount within the range of l-200 grams per liter and preferably within the range of 20-200 grams per liter. The amount of sulphuric acid should not exceed 50 grams per liter and it is preferred to formulate the aqueous electrolytic bath with an amount of sulphuric acid within the range of 3-10 grams per liter. Gold layers of greater harness can be obtained when the amount of sulphuric acid is maintained within the range of -10 grams per liter.

The anodic oxidation of the aluminum or alloy of aluminum in the electrolytic bath can be carried out with direct current or with alternating current or by superimposing an alternating current on a direct current, in a system generally referred to as pulsating current. Current densities can be used ranging from table,

l to 10 A/dm and preferably within the range of 1.5 to 4.5 A/dm. The temperature of the electrolyte can be varied during operation within the range of 0 to 70 C. but it is preferred to make use of a temperature within the range of 15 to 30 C.

Under these conditions, a suitable coating can be obtained within a time ranging from 1 to 120 minutes depending somewhat upon the thickness of the coating desired and the current density. During the electrolytic treatment, it is desirable to maintain the electrolytic bath in a state of agitation either by the use of mechanical stirring means or by the use of gaseous streams.

The electrolytic treatment should be preceded by the usual steps for preparation of the sheet or shaped metal product such as by the processing steps of polishing, degreasing and pickling, and the electrolytic treatment should be followed by the usual procedures of rinsing, sealing and perhaps brightening.

The resulting product will have a layer of uniform color overthe entire exposed surface and in which a color of sufficient intensity will be available in a layer having a thickness in the order of about 10 microns but in which the intensity of color can be further increased in proportion to the increased thickness of the color layer which may be as great as 30 microns or more.

The following examples are given by way of illustration, but not by way of limitation of our invention:

EXAMPLE 1 in this series of examples the sheets or shaped elements of aluminum or alloys of aluminum, as indicated in the following were polished mechanically, degreased with trichlorethylene, and pickled by soaking for 10 minutes in a 20 to 25 C. bath containing, per liter, 300 cc. of nitric acid, 10 grams of sodium fluoride and 1 cc. of a surface active agent such as an alkyl sulphonate, commercially known as CELANOL A. After rinsing, the elements were immersed in the electrolyte in the form of an aqueous solution containing 5 grams of sulphuric acid per liter and :1 grams of chromotropic acid per liter in which n 5, 20 and grams. The current density was kept constant at the beginning at 2.5 A/dm while the voltage at the terminals was progressively raised to 60 volts and thereafter a constant voltage of 60 volts was employed. The temperature was maintained at 20 C. In each case, the operation was carried out on three identical samples in which the first was withdrawn after 20 minutes, the second after 30 minutes, and the last after 45 minutes of electrolysis with the result that an average thickness of an oxide layer of 5 to 8 microns was produced on the first sample, 12 to 15 microns on the second sample, and 20 to 30 microns on the third sample.

The following table indicates the color which is obtained in V which the colors difier according to the composition of the metal that is treated:

TABLE 1 Alloy Color A1 99.99% pale gold A1 99.5% greyish beige A-GS Si 0.4%

Mg 0.47% bronze A-SG Si 1.1% bronze-grey Mg 1.0% A-G 3 Mg 3% brown A-Z4G Zn 3% Mg 2% chestnut-black A-S5G Si 5% Mg 0.5% grey-black A-U4G Cu 4% Mg 0.5% grey-reddish brown The color of all of the samples was very uniform and the intensity of color increased in proportion to the increased thickness of the oxide layer. Very little difference is found to exist between the results when the content of chromotropic acid in the bath is varied in the amounts of 5, 20 and 100 grams per liter. This is an important factor because it becomes superfluous to maintain a constant analysis of the composition of the bath for controlling the content of chromotropic acid and it also favors the ability to produce uniform results from piece to piece.

EXAMPLE 2 Sheets of aluminum alloy containing 3 percent magnesium, previously degreased with trichlorethylene and then cleaned for minutes at 10 to 25 C. in a bath containing, per liter, 300 cc. of nitric acid, 10 grams of sodium fluoride and 1 cc. of alkyl sulphonate, followed by rinsing, are anodized in an aqueous bath containing the following ingredients and under the following conditions:

100 grams per liter 2 grams per liter chromatropic acid sulphuric acid The electrolysis was operated at 2.5 A/dm" up to 60 volts and then with a substantially constant voltage of 60 volts and at a temperature of 22 C. After anodization for a period of 30 minutes, a deep brown oxide layer having a thickness of 20 microns was obtained.

EXAMPLE 3 EXAMPLE 4 Sheets of the same alloy as in Example 2 were subjected to the same surface preparation and then were anodized in an aqueous bath containing 50 grams per liter of G acid and 7 grams per liter of sulphuric acid and operated at 4 A/dm up to 60 volts and then at a constant voltage of 60 volts with a bath temperature at 20 C. After anodizing for minutes, a deep brown oxide layer of 15 microns was obtained.

EXAMPLE 5 Shaped elements of aluminum alloy containing 1.10 percent by weight silicon, 1.00 percent by weight magnesium and 0.44 percent by weight manganese, the remainder aluminum, were subjected to the same surface treatment as in Example 2 and then anodized in an aqueous bath containing 100 grams per liter of chromotropic acid and 7 grams per liter of sulphuric acid and operated at 2.5 A/dm up to 60 volts and then at a constant voltage of 60 volts, and at a temperature of 22 C. After anodizing for 30 minutes, a black oxide layer of 12 microns in thickness was developed.

\ EXAMPLE 6 Shaped elements formed of an aluminum alloy containing 0.40 percent by weight of silicon and 0.47 percent by weight of magnesium, the remainder aluminum, were subjected to the same surface preparation as in Example 2 and then anodized in an aqueous bath containing 40 grams per liter of H Acid and 9 grams per liter of sulphuric acid and operated at 3 A/dm up to 60 volts and thereafter at a constant voltage of 60 volts with a temperature of C. After 20 minutes, there was obtained a bronze colored oxide layer of 15 microns in thickness.

Bath composition of the types described find excellent use when it is desired to produce aluminum and alloys of aluminum having deeply colored shades on the surfaces thereof.

Lighter shades can be produced by modification to raise both temperatureor to reduce current density.

citraconic acid.

Such mixtures of naphthalene sulphonicacid, aliphatic acid and sulphuricacid enables the electrolytic treatment to be carried out at lower voltage and it enables the naphthalene sulphonic acid to be used in lower concentrations in the bath.

The amount of aliphatic acid use will depend somewhat on the nature of the acid and the effect desired to be secured but, in general, it is'desired to make use of an electrolytic bath in which the aliphatic acid is present in an amount within the range of 10-100 grams per liter. The amount of naphthalene sulphonic acid can be reduced to less than grams per liter and preferably within the range of 50-100 grams per liter or up to saturated solution when use is made of a naphthalene sulphonic acid containing an amino group.

Generally colored oxide layers having a thickness within the range of 10-30 microns should be produced. The colored layer that is formed is characterized by good hardness and excellent'light and weather resistance.

The following examples are given by way of illustration, but not by way of limitation, of the further practice of this invention:

EXAMPLE 7 An aluminum alloy containing about 0.6 percent by weight of silicon and 0.7 percent by weight of magnesium is anodized in an aqueous bath containing in solution:

chromotropic acid 50 g/l oxalic acid 10 g/l sulphuric acid 3.5 g/l Five tests were carried out with different current densities The aluminum oxide layer is hard, the color is very uniform and has excellent resistanceto ultraviolet rays.

EXAMPLE 8 A sheet of commercial aluminum (99.5 percent Al) is anodized in an aqueous bath containing in solution:

chromotropic acid acetic acid sulphuric acid so g/l 100 g/l 7 8/1 The anodizing is carried out at 30 C. with a current density I of 1.5 aldm After 30 minutes, an oxide layer is obtained having a thickness of 15 microns and a light bronze color. The shade is very unifonn and resistant to ultraviolet rays.

EXAMPLE 9 Shaped elements of aluminum alloy having a composition the same as that in Example 7 are anodized for 30 minutes at 25 C. with a current density of 1.5 a/dm' in a bath containing chromotropic acid, maleic acid and sulphuric acid. Several tests were carried out in which the proportions of acids were varied as set forth in the following table:

TABLE III Electrolysis on voltage Color No. of Composition of bath commence-on comof the test ment pletion layer 1 chromotropic l gll maleic 50 41 V 50 V medium sulphuric 3.5 bronze 2 chromotropic 100 g/l bronze maleic 75 40.5 50 lighter sulphuric 3.5 than 1 3 chromotropic I00 gll bronze maleic I00 40.5 52 lighter sulphuric 3.5 than 2 4 chromotropic 25 g/l no anodization maleic I00 corrosion of the metal sulphuric 3.5

5 chromotropic 50 gll deep maleic I00 42.5 48.5 bronze sulphuric 3.5

6 chromotropic 50 g/l maleic 100 34.5 42.5 light sulphuric 7 bronze It will .be seen that with a bath containing 100 grams of chromotropic acid and 3.5 grams of sulphuric acid per liter, the increase in maleic acid content lightens the shade slightly but does not alter the electrolysis voltage necessary for maintaining the current density at 1.5 aldm The presence of maleic acid in the amount of 100 g/l permits the amount of chromotropic acid in the bath to be reduced to about 50 gll. A content of sulphuric acid of the order of 7 g/l is preferred for reducing the electrolysis voltage.

EXAMPLE 10 This example is intended to show the extent of the range of colors which can be obtained on aluminum or on alloys of alu- EXAMPLE I l A shaped element of aluminum alloy containing l percent silicon, 0.5 percent magnesium and 0.5 percent manganese (by weight) is anodized in a bath formed of an aqueous solution of:

l-naphthol-8-amino-3, 6-sulphonic acid (monosodium salt) 25 gll acetic acid 80 cell sulphuric acid 5 gll Anodization was carried out with a current density of 2.5 a/dm at 20 C. for minutes. A black oxide layer of uniform appearance, having a thickness of 20 microns, is obtained.

EXAMPLE 12 A shaped element of aluminum-magnesium-silicon alloy containing about 0.5 percent silicon and 0.5 percent magnesium (by weight) is anodized in a bath formed of an aqueous solution of:

l-naphthol-8-amino-3, 6-sulphonic acid 10 g/l propionic acid I00 g/l sulphuric acid 6 g/l EXAMPLE 13 A shaped element of aluminum-magnesium-silicon alloy containing 0.5 percent silicon and 0.5 percent magnesium (by weight) is anodized in a bath formed of an aqueous solution of:

2-naphthol-6, 8-disulphonic acid 60 gll oxalic acid gll sulphuric acid 3 gll The anodization is carried out at 25 C. with a current density of L5 a/dm. After 45 minutes, a light bronze colored layer mil'lum with a bath of given composition merely by varying the 45 having a thickness of l 8 microns is obtained. The shade is very current density and temperature. The aqueous bath was forif and resistant to ultraviolet rays mulated of the following in solution:

EXAMPLE l4 chromotro ic acid 50 /l maleic 100 i 50 Shaped elements of alummum-magnesium-srllcon alloy havsulphuric acid 7 g/l mg the composition of Example 7 are anodized in an aqueous TABLE IV electrolysis voltage on comthickcurrent temperduration mence on comness of Nature of metal density alure "C. minment pletion the layer color 99.5% aluminum l.5 a/dm 40 33.5 V 36 V 18 very sheets. 2.5 25 20 37.5 43 16 p. Myrna 25" 30 42.5 48.5 15p medium 2.5 20 30 40 60 22 deep Al 1.5 30" 40 35 40.5 16 u very shaped 1.5 25" 20 34.5 42.5 13 [L light elements 2 20 30 37 42.5 17 .1. medium Si 0.6% MG 0.7% 2.5 20 30 34 22 deep AAI sheets 2.5 20 45 38 25 p. black Mg 0.5% Mn 0.5% Al shaped elements Si 1% 2.5 20 45 40 60 20 p. black Mg 0.5% Mn 05% It will be apparent that, with the same bath composition, it is possible to obtain, on aluminum or on aluminum-magnesiumsilicon alloys, shades which extend from very light bronze to deep bronze and that with a]uminum-manganese-magnesium or aluminum-silicon-manganese-magnesium alloys, it is possible to obtain black shades.

bath containing:

2-naphthol-6, 8-disulphonic acid (dipotassium salt) oxalic acid sulphuric acid 50 gll 50 g/l 3.5 g/l EXAMPLE is A commercial aluminum sheet (99.2 percent aluminum) is anodized in a bath formed of an aqueous solution of:

2-hydroxynaphthalene-3, 6-disulphonic acid 50 gll acetic acid 100 g/l sulphuric acid 7 g/l The anodization is carried out at 20 C. with a current density of 2.5 aldm After 30 minutes, a bronze colored layer having a thickness of 20 microns is obtained. The shade is uniform and resistant to ultraviolet rays.

EXAMPLE 16 chromotropic acid 50 g/l succinic acid 65 'g/l sulphuricacid g/l The anodization is carried out at 20 C. with a current density of 2.5 a/dm for 45 minutes. An anodized layer having a thickness of microns is obtained which is black in color.

The shade is very uniform and resistant to ultraviolet rays.

EXAMPLE 17 Sheets of an alloy consisting of aluminum with 3 percent magnesium are anodized in an aqueous bath containing:

chromotropic acid 50 g/l citraconic acid 100 g/l sulphuric acid 7 g/l Anodization is carried out for minutes at a voltage of volts. A uniform brown anodized layer is obtained which is resistant to ultraviolet rays.

It will be understood that changes may be made in the details of formulation and conditions for anodization without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. In a process for producing a colored layer on aluminum and alloys of aluminum by anodic oxidation in an electrolytic bath, the steps of mounting the aluminum article as an anode in the electrolytic bath formulated of an aqueous solution containing as essential ingredients the combination of sulphuric 6. The process as claimed in claim 1 which includes a step in maintaining the bath at a temperature within the range of l5-30 C during electrolysis.

7. A process as claimed in claim 1 in which the current that is passed between the electrodes through the bath is direct current.

8. A process as claimed in claim 1 in which the current that is passed between the electrodes through the bath is altemating current.

9. A process as claimed in claim 1 in which the current that is passed between the electrodes through the bath is pulsating current.

10. A process as claimed in claim 1 in which the current is passed at a current density of l-lO A/dm.

1 11. A process as claimed in claim 1 in which the current is passed at a current density of 1.4 to 4.5 A/dm.

12. An electrolytic bath for coloring aluminum and alloys of aluminum by anodization comprising ,an aqueous solution containing the combination of a naphthalene sulphonic acid' selected from the group consisting of 2'-naphthol-6, 8- disulphonic acid, 2-naphthol-3, o-disulphonic acid, 1,8- dihydroxynaphthalene-3, G-disulphonic acid and l-naphthol- 8-amino-3, ti-disulphonic acid, an aliphatic acid of less than five carbon atoms and sulphuric acid.

13. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or alkali metal salt is present in an amount within the range of 10 to 200 grams per liter.

14. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or alkali metalsalt is present in an amount within the range of to 100 grams per liter.

15. An electrolytic ath as claimed in claim 12 in which the naphthalene sulphonic acid or salt contains an amine group, and is present in the bath in an amount to saturate the solution.

16. An electrolytic bath as claimed in claim 12 in which the aliphatic acid is present in an amount within the range of 10 to 100 grams per liter.

17. An electrolytic bath as claimed in claim 12 in which the sulphuric acid is present in an amount within the range of 3 to 10 grams per liter.

18. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or its alkali metal salt is present in an amount within the range of 10 to 200 grams per liter, the aliphatic acid is present in an amount within the range of 10 to I00 grams-per liter and the sulphuric acid is present in an amount within the range of 3 to 10 grams per liter.

7 19. An electrolytic bath as claimed in claim 12 in which the aliphatic acid is selected from the group consisting of saturated monocarboxylic acids, saturated dicarboxylic acids and ethylenically unsaturated dicarboxylic acids.

20. An electrolytic bath as claimed in claim 12 in which the aliphatic acid is selected from the group consisting of acetic acid and a compound selected from the group consisting of 2- naphthol-6, 8-disulphonic acid, 2-naphthol-3, 6-disulphonic acid, 1, 8-dihydroxynaphthalene-3, 6-disulphonic acid and lnaphthol-8-amino-3, 6-disulphonic acid, and water soluble salts thereof in which the sulphuric acid is present in an amount within the range of l-200 grams per liter. while the disulphonic acid derivative is present in an amount within the range of 1-200 grams per liter, and passing current between the electrodes through the bath.

2. A process as claimed in claim 1 in which the sulphuric acid is present in an amount up to 50 grams per liter.

3. A process as claimed in claim 1 in which the sulphuric acid is present in an amount from 3-10 grams per liter.

4. A process as claimed in claim 1 in which the disulphonic acid component is present within a range of 1-20 grams per liter.

5. The process as claimed in claim 1 which includes a step in maintaining the bath at a temperature within the range of 0 to 70 C during electrolysis.

acid, propionic acid, oxalic acid, succinic acid, maleic acid, i'taconic acid and citraconic acid.

21. In the method of coloring aluminum and alloys of aluminum by anodizing, the step of immersing the aluminum or alloy of aluminum in an electrolytic bath as claimed in claim 12, as one electrode, providing another electrode in the bath and passing an electric current through the bath between said electrodes.

22. The method as claimed in claim 21 which includes the step of degreasing the aluminum base alloy before anodization.

' 23. The method as claimed in claim 21 which includes the step of sealing the anodized surface with an boiling aqueous solution of an organo metallic salt.

24. The method as claimed in claim 23 in which the salt dissolved in the sealing solution is nickel acetate present in the solution in an amount within the range of 0.5 to 1 gram per liter.

25. The method as claimed in claim 21 in which a color ranging from light to deep bronze is produced on aluminum and on aluminum-magnesium-silicon alloys and a black color is produced on aluminum-silicon-magnesium-manganese and aluminum-magnesium-manganese alloys. 

2. A process as claimed in claim 1 in which the sulphuric acid is present in an amount up to 50 grams per liter.
 3. A process as claimed in claim 1 in which the sulphuric acid is present in an amount from 3-10 grams per liter.
 4. A process as claimed in claim 1 in which the disulphonic acid component is present within a range of 1-20 grams per liter.
 5. The process as claimed in claim 1 which includes a step in maintaining the bath at a temperature within the range of 0* to 70* C during electrolysis.
 6. The process as claimed in claim 1 which includes a step in maintaining the bath at a temperature within the range of 15*-30* C during electrolysis.
 7. A process as claimed in claim 1 in which the current that is passed between the electrodes through the bath is direct current.
 8. A process as claimed in claim 1 in which the current that is passed between the electrodes through the bath is alternating current.
 9. A process as claimed in claim 1 in which the current that is passed between the electrodes through the bath is pulsating current.
 10. A process as claimed in claim 1 in which the current is passed at a current density of 1-10 A/dm2.
 11. A process as claimed in claim 1 in which the current is passed at a current density of 1.4 to 4.5 A/dm2.
 12. An electrolytic bath for coloring aluminum and alloys of aluminum by anodization comprising an aqueous solution containing the combination of a naphthalene sulphonic acid selected from the group consisting of 2-naphthol-6, 8-disulphonic acid, 2-naphthol-3, 6-disulphonic acid, 1,8-dihydroxynaphthalene-3, 6-disulphonic acid and 1-naphthol-8-amino-3, 6-disulphonic acid, an aliphatic acid of less than five carbon atoms and sulphuric acid.
 13. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or alkali metal salt is present in an amount within the range of 10 to 200 grams per liter.
 14. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or alkali metal salt is present in an amount within the range of 50 to 100 grams per liter.
 15. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or salt contains an amine group, and is present in the bath in an amount to saturate the solution.
 16. An electrolytic bath as claimed in claim 12 in which the aliphatic acid is present in an amount within the range of 10 to 100 grams per liter.
 17. An electrolytic bath as claimed in claim 12 in which the sulphuric acid is present in an amount within the range of 3 to 10 grams per liter.
 18. An electrolytic bath as claimed in claim 12 in which the naphthalene sulphonic acid or its alkali metal salt is present in an amount within thE range of 10 to 200 grams per liter, the aliphatic acid is present in an amount within the range of 10 to 100 grams per liter and the sulphuric acid is present in an amount within the range of 3 to 10 grams per liter.
 19. An electrolytic bath as claimed in claim 12 in which the aliphatic acid is selected from the group consisting of saturated monocarboxylic acids, saturated dicarboxylic acids and ethylenically unsaturated dicarboxylic acids.
 20. An electrolytic bath as claimed in claim 12 in which the aliphatic acid is selected from the group consisting of acetic acid, propionic acid, oxalic acid, succinic acid, maleic acid, itaconic acid and citraconic acid.
 21. In the method of coloring aluminum and alloys of aluminum by anodizing, the step of immersing the aluminum or alloy of aluminum in an electrolytic bath as claimed in claim 12, as one electrode, providing another electrode in the bath and passing an electric current through the bath between said electrodes.
 22. The method as claimed in claim 21 which includes the step of degreasing the aluminum base alloy before anodization.
 23. The method as claimed in claim 21 which includes the step of sealing the anodized surface with an boiling aqueous solution of an organo metallic salt.
 24. The method as claimed in claim 23 in which the salt dissolved in the sealing solution is nickel acetate present in the solution in an amount within the range of 0.5 to 1 gram per liter.
 25. The method as claimed in claim 21 in which a color ranging from light to deep bronze is produced on aluminum and on aluminum-magnesium-silicon alloys and a black color is produced on aluminum-silicon-magnesium-manganese and aluminum-magnesium-manganese alloys. 